This invention relates to an apparatus for applying coatings to articles, and more particularly for applying a fog/spray to article surfaces which, because of inaccessibility, cannot otherwise be conveniently coated. For example, in the automobile industry the exterior surfaces of automobile bodies and frames may be readily coated using spray painting techniques which are well known in the art. However, automobile body members have a relatively large number of interior recesses which are inaccessible to the usual type of spray applicators, and which require special techniques if a complete and uniform coating is to be applied over all interior surfaces.
In general, there are two approaches to applying coatings to the surfaces of articles by means of spraying: hydraulic (airless) spraying, and air atomization (air) spraying. These two spraying techniques use basically different approaches for the atomization of the fluid being sprayed. In an airless spraying technique the fluid to be sprayed is placed under high hydraulic pressure, usually well in excess of 100 pounds per square inch (psi), and the pressurized fluid is forced through a very small orifice. The result is that the fluid particles atomize into very fine droplets and may be sprayed in this manner. Air spray techniques also utilize a very small orifice, but the fluid to be sprayed is pressurized at relatively low pressure, i.e., under 100 psi, in being forced through the orifice. Exterior to the orifice, a pressurized air stream is directed toward the emitted fluid particles and this pressurized air causes the particles to further atomize and thereby provides the desired degree of atomization for spraying.
The present invention utilizes the combination of these techniques to provide a fog/spray, wherein the fluid particles are very finely atomized into a mist or fog which may be ejected into an enclosed recess so as to settle over all interior surface areas of the recess. Conventional spraying techniques utilize apparatus for developing a spray pattern of predetermined shape, whereas the present invention is primarily concerned with the production of a fog and does not utilize a spray pattern except to the extent that the directional output of the fog may be controlled through suitable openings.
Because fog/spray devices are most useful for the application of coatings to interior and hard to reach recesses, they usually require an elongated tubular spray nozzle having end openings which can be inserted deep into interior recesses. The use of these elongated tubular applicators presents special problems to the aforementioned two types of conventional spraying. The technique of airless spraying would require the placement of an atomization spray tip at the end of this long applicator, and would therefore require that the applicator be sufficiently strong to withstand the dangerous high fluid pressures necessary in this technique. Because of space constraints, the fluid valve which would control the pressurized fluid emitted into the applicator must be placed at the opposite end of the applicator. This results in a column of fluid being held in the applicator between the fluid valve and spray tip, and causes the spray tip to emit droplets of fluid after the valve is shut off. This well known phenomenon is called "spitting," and occurs any time a pressurized fluid reservoir is permitted to be developed between the fluid valve and the spray tip. Air spray techniques cannot be used with an elongated applicator because air spray requires both air and fluid to be supplied to the end of the applicator. This would require a bulky air delivery and valving system which would restrict the use of the device to only larger recesses. If the air stream was coupled into the opposite end of the wand the air pressure used for atomization would not be enough to also convey the atomized spray through the wand. If the air pressure were increased, a condition would soon be reached whereby the atomization air pressure would exceed the fluid pressure and would impede fluid atomization. Thus, the present invention utilizes air pressures for ait atomization which exceed the pressures usually used in the prior art for this purpose (20-50 psi), and utilizes hydraulic fluid pressures in combination therewith which exceed the hydraulic fluid pressures usually used with air spray devices, but are lower than the hydraulic pressures usually used with airless spray devices (700-1000 psi).
Fog/spray devices are known in the prior art, as evidenced by the disclosure of U.S. Pat. No. 3,202,363, issued Aug. 24, 1965. This patent discloses a fog/spray device having a spray applicator gun with an interior chamber and valving arrangement to permit the mixing of pressurized fluid and air to create a fog. The patent disclosure describes a piston valve which is actuated by hydraulic fluid pressure working against a compression spring, and which lifts a poppet valve from its seat to admit air into a chamber surrounding the atomization spray tip. The pressurized air combines with the atomized fluid to create a fog which is transported through an elongated wand to the interior surface to be coated. As compared to this prior art patent, the present invention utilizes a novel approach for controlling the valving and mixing of the pressurized air in combination with the atomized fluid.